The invention relates to a heating apparatus for heating sheet material before hot-forming thereof, comprising two members for contacting the two surfaces of the sheet material for heat transfer, one of the members comprising a layer of elastic material facing the sheet material.
A heating apparatus of this kind is known from the Swiss Pat. No. 474 347. The member comprising the layer is formed as a support A, the layer comprising a smooth surface. The German laid open print No. 27 24 957 discloses a heating apparatus for heating sheet material before the hot-forming thereof, comprising a member for contacting the sheet material for heat transfer with a relief-type surface. The sheet material is pressed onto the contacting surface by air-pressing the sheet material to the contact area.
As far as here and further below the term of the relief-type surface is used, it should be understood that, as will be shown by the embodiments described further below, surfaces comprising parts more or less projecting in a raised manner from a plane as well as surfaces comprising a plurality of depressions together with surrounding regions are meant.
In the forming station of a packaging machine a heating apparatus is provided comprising two plate-shaped members having plane surfaces for contacting the sheet material to be heated. One of the plates is stationarily disposed immediately above the sheet material, whereas the second plate is upwards and downwards movable relative to the first plate. In operating condition the second plate is moved towards the first plate in such a manner that the sheet material is interposed between both plates with a predetermined pressure. Both plates are heated and temperature-controlled by means of respective apparatus and transfer heat to the interposed sheet material. In the next operating cycle the lower plate is lowered and the pre-heated sheet material section is moved on into the actual forming tool for forming. If the sheet material to be formed is a hard sheet material, e.g. made of PVC, polypropylene or polystyrene, then this sheet material may have a thickness up to the order of 1 mm. In this case the surfaces of the sheet material are not completely plane parallel, but the thickness of the sheet material has tolerances of about 1/10 of the total thickness in individual regions. If the two plane heating plates are moved towards each other, then air is included in those regions which are depressed due to the unevenesses of the sheet material. The thus formed air cushions prevent an intimate contact between the heating plate and the sheet material and hence a heat transfer to the sheet material. This leads to an ununiform heating in different regions of the sheet material. Thus an unobjectionable deformation of the sheet materials is impossible in particular where the sheet materials lack a sufficiently large tolerance region for the forming temperature.